Spraying cold makeup propane into vapor space of dewaxing chiller

ABSTRACT

Cold makeup liquid normally gaseous dewaxing solvent is introduced directly into the chiller of a dewaxing plant by spraying it into the vapor space of the chiller in droplets having a size between 1,000 and 6,000 microns, which are rapidly heated by the ascending solvent vapors from the liquid solvent in the chiller thus preventing shock chilling.

United States Patent Inventors James D. Bushnell Berkeley Heights; Richard K. Neeld, Plainfield; Roy J. Lamm, Chester, all of NJ.

Appl. No. 24,529

Filed Apr. 1, 1970 Patented Nov. 23, 1971 Assignee Esso Research and Engineering Company SPRAYING COLD MAKEUP PROPANE INTO VAPOR SPACE OF DEWAXING CHILLER 5 Claims, 1 Drawing Fig.

U.S. Cl 208/35, 208/33, 208/37 Int. Cl C01 g 43/08 Field ofSearch 208/35, 33, 37

DRY PROPANE FROM DEWAXED OIL SOLVENT RECOVERY HIGH PRESSURE FLASH TOWER PROPANE -/4 STORAGE CHILLER MIXING COLUMN [56] References Cited UNITED STATES PATENTS 2,079,182 5/1937 Petty et al 208/35 2,083,700 6/1937 Forrest et al 208/35 2,143,872 1/1939 Forrest et al 208/35 2,903,410 9/1959 Basch 208/35 2,909,475 10/1959 Bushnell 208/35 Primary ExaminerHerbert Levine Attorneys-Pearlman and Stahl and C. Di Stores ABSTRACT: Cold makeup liquid normally gaseous dewaxing solvent is introduced directly into the chiller of a dewaxing plant by spraying it into the vapor space of the chiller in droplets having a size between 1,000 and 6,000 microns, which are rapidly heated by the ascending solvent vapors from the liquid solvent in the chiller thus preventing shock chilling.

w WASH 5895 e225 PROPANE ROTARY FILTER DEWAXED OIL SPRAYING COLD MAKEUP PROPANE INTO VAPOR SPACE OF DEWAXING CI-IILLER BACKGROUND OF THE INVENTION This invention relates to improvements in the art of solvent dewaxing of petroleum oil fractions and more particularly to an improved solvent dewaxing process in which the oil is cooled or chilled by the evaporation of the solvent.

The solvent dewaxing of waxy hydrocarbon fractions by the use of autorefrigeration is well known in the art. In such processes the oil to be dewaxed is mixed with a solvent in which the wax is insoluble at low temperature and the resulting solution is cooled by the evaporation of the solvent until wax crystals are precipitated from solution. The wax crystals thus formed are separated from the slurry by means of filtration. The wax is then further purified while the solvent is recovered for reuse and the dewaxed oil is sent to other refinery processes.

In the process described above it is often desirable to add cold solvent into the chiller to replace solvent vaporized therefrom. If this cold solvent were introduced directly into a body of warm waxy oil solution there would be shock chilling which would result in the formation of fine amorphous, nonfilterable and nonsettleable wax crystals. Therefore it has been considered necessary in the art to introduce the cold makeup solvent into a contacting chamber placed above the chiller and filled with contact elements such as bubble trays, discs and doughnuts and the like. The ascending warm solvent vapors formed by the evaporation of the solvent in the chiller condense in the newly added cold solvent, warming it up so that by the time it reaches the liquid body in the chiller it is at substantially the same temperature as that body. At the same time solvent vapors are removed from the chiller in amounts regulated to obtain the desired rate of cooling. The process described above is exemplified by such patents as US. Pat. No. 2,083,700 to Forrest et al., US. Pat. No. 2,l43,872 to Forest et al., U.S. Pat. No. 2,903,410 to Basch et al., and US. Pat. No. 2,909,475 to Bushnell.

While the above process has worked well in practice, the use of a contacting tower above the chiller is an additional expense and if it could be eliminated important savings could be realized. In addition to the investment cost of the contacting vessel itself, the weight of metal in it, which must be heated up and cooled down each chilling cycle and the volume of gas it contains contribute to thermodynamic inefficiency. In a typical case this inefficiency contributes about 9 percent increased load to the refrigeration compressor and the subsequent solvent condenser, so that both of these need to be larger, with high investment; and the increased compression and condensing load consumes greater amounts of utilities.

SUMMARY OF THE INVENTION It has now been found that the contacting tower described above can be eliminated by spraying the cold solvent directly into the vapor space above the liquid solvent-oil mixture. By introducing the cold makeup solvent through sprays, fine droplets are produced allowing the sensible and latent heat of vaporization of the solvent vapors from the evaporating solvent to preheat the inlet solvent. This avoids mixing the colder makeup solvent with the contents of the chiller, thereby eliminating shock chilling of the wax. Spray nozzles are uniquely well suited to this operation because:

They provide very uniform liquid distribution over the entire cross-sectioned area of the chiller.

They produce fine drops, thereby increasing interfacial area which maximizes liquid/vapor heat transfer, without making them so fine that they are entrained out of the chiller with the vapor.

The liquid droplets settle into the bulk liquid unifon'nly over the surface, thus avoiding concentration gradients which would exist if liquid from a contractor trap were to enter in bulk via a downcomer.

BRIEF DESCRIPTION OF THE DRAWING The drawing sets forth in diagrammatic form a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWING 900 F. if microcrystalline wax is to be produced. A typical fraction for the production of parafi'm wax might be a waxy distillate having a viscosity of about 45 S.S.U. at 210 F. and derived from East Texas Crude. Liquid propane stored under a pressure in the range of about 170 to 250 p.s.i.g. in storage tank 4, is pumped by pump 5 through line 6 and preheater 8 and introduced into the hydrocarbon stream in line 1 just before the said stream is introduced into mixing column 3. The liquid propane and hydrocarbon fraction are introduced into mixing column 3 through line I and are thoroughly mixed by passing through a series of orifice plates (not shown). The amount of propane to be mixed with the oil varies with the viscosity of the feed stock and normally ranges between 1.5 volumes to 3.5 volumes per volume of feed. Lighter feeds require less propane.

The solution of oil and propane is conducted from mixing column 3 through line 7 to heat exhanger 30 where it is cooled by water or another cool stream to a temperature ranging between about 50 F. and F. preferably, preferably about 70 F. and thence into warm solution tank 9, where it is stored for introduction into the chilling step of the process. The warm solution of propane and oil is fed from the warm solution tank 9 through line 10 alternately into one of two batch chillers 11 at a temperature between 90 F. and and 50 F., preferably 70 to 80 F. and a pressure between and 200 p.s.i.g. preferably -450 p.s.i.g. Only the batch chiller is shown in the drawing but it will be understood that at least two chillers are normally used to assure continuity of operation.

The chilling of the solution and oil takes place in insulated chiller II which is equipped with a means for withdrawing vapor therefrom, thereby regulating the pressure therein. The chiller is preheated by hot propane vapor to about 60 E, which is near to the temperature of the solution of propane and oil in the warm solution tank 9, before the oil-propane solution is admitted. This preheating is necessary to prevent shock chilling of the solution by the cold metal wall of the empty chiller and the precipitation of very fine crystals which are difiicult to filter. The preheating line is not shown.

After the solution has been introduced into the preheated chiller 11, the chiller pressure is gradually reduced by the withdrawal of vapor through line 12. The decrease in pressure causes liquid propane to evaporate from the solution, thereby cooling the solution. The dilution of the solution is maintained by the addition of makeup liquid propane from the propane storage drum 4 and/or the dewaxed oil recovery high-pressure flash drum overhead. This makeup propane is commonly cooled by exchange with cold streams from the filter such as wax slurry or filtrate, in order to recover the refrigeration in these latter streams. However, the cold propane in bulk cannot be mixed directly with the warm solution or slurry in the bottom of the chiller, or shock chilling would result. Instead, the makeup propane is passed through line 18, heat exchange 19 (where it is cooled) and nozzles 16 which spray the propane into the vapor space of chiller 11, forming fine droplets which are rapidly heated by the warm propane vapors leaving the surface of the propane-oil solution as a result of the reduction in pressure. By this means the colder makeup propane is kept from mixing with the liquid contents of the chiller thereby avoiding shock chilling of the wax. Part of the propane vapor evaporated from the batch is condensed by the cold makeup and returns to the batch as warm liquid. This phenomenon lowers the required quantity of makeup propane. The droplets produced must be kept in the range of about 1 ,000 and 6,000 microns in size. If they are much larger, insufficient heat transfer may occur. If they are much smaller, they may be entrained with the gas stream leaving through line 12. This size range usually occurs at a pressure drop through the nozzle of about 5 to 45 p.s.i. Care must also be taken to insure that the spray is uniformly distributed across the chillers entire cross-sectional area in order to maximize heat transfer. The chilling vessels may be vertical cylindrical, as shown in the drawing, horizontal cylindrical, or spherical in shape.

The removal of vapors from the chiller, together with the addition of cold propane, causes a gradual lowering of temperature. By employing this method of cooling a constant diluent ratio is maintained. However if desirable the diluent ratio may increase or decrease, depending on the amount of liquid propane introduced through spray nozzles 16 as compared with the amount of propane vapors withdrawn through line 12.

The chilling rate during the process is carefully controlled so that the initial rate is not greater than approximately 7 F. per minute. The complete chilling cycle normally takes from 30 to 35 minutes. While the wax crystals are being precipitated from solution in one chiller, the other chiller is being emptied, preheated and refilled with solution from the warm solution tank. This permits chilling to be carried on continuously.

Upon completion of precipitation of the wax crystals in chiller 11, the slurry of wax, oil and liquid propane is discharged from the chiller through line 20 into filter feed tank 21. This slurry is at a temperature of about 35" F. and approximately atmospheric pressure. The temperature and pressure level is maintained by the withdrawal of propane vapor from the filter feed tank through line 22. The cold slurry is pumped from the feed tank through line 23 by pump 24 into rotary filter 25.

The rotary filter 25 is of the pressure type and is operated with a pressure in the filter housing of about 5 p.s.i. higher than the filtrate receiver in order to provide the pressure differential necessary for filtration. Crude wax is filtered from the slurry, washed with cold propane supplied through line 26, blown with cold propane gas supplied through line 27, and discharged into trough 28. The dewaxed oil and solvent are discharged from the filter through line 29. It is generally desirable to use more than one filter in order to achieve a balanced process.

Following the filtration operation the crude wax is further processed for the removal of solvent in conventional equipment not shown in the drawing and then may be further refined. The dewaxed oil and solvent are separated by conventional evaporation and stripping operations not shown. The solvent is then re-recycled to propane storage for reuse in the process and the dewaxed oil is sent to other product tankage for blending into finished lubricating oil.

It is to be understood that the invention is not limited to the specific arrangement of equipment disclosed above to illustrate one form of the invention and that the principles disclosed may be embodied in other solvent dewaxing processes.

While the above description has been given disclosing propane as the refrigerative solvent, it is to be understood that other refrigerants are contemplated. Suitable autorefrigerants include any liquid normally gaseous C through C hydrocarbon, such as ethane, ethylene, propane, propylene, butanes, butylenes, and mixtures thereof either alone or in combination with halogenated substitution products of methane and ethane, normally gaseous ethers, sulfur dioxide, methyl amine, etc. Solvent mixtures such as 20 percent acetone and 80 percent propylene are also satisfactory. Propane and propylene are preferred. These solvents, being very fluid at low temperatures, reduce the viscosity of even the heaviest oils to such an extent that satisfactory filter rates and yields can be obtained at the low temperatures necessary to make low-pour point oils provided that chilling conditions are maintained which will generate wax crystals of satisfactory size and form.

The invention may be employed for the dewaxing of any wax-bearing oil, but is particularly useful in the dewaxing of petroleum distillates or petroleum residues from which asphalt has been separated. The invention is also useful on residual stocks containing small amounts of asphalt, particularly when the asphalt itself acts as a filter aid or settling aid and helps to carry down the wax in readily separable form.

EXAMPLE To further illustrate the invention and to demonstrate the commercial practicality of the technique of spraying the makeup refrigerant into the vapor space of the chiller without shock chilling the wax, a 5 day test was made in commercial equipment of the process of the invention using as feed stock a 600 neutral stock made up of 72 percent of a stock of Aramco origin and 28 percent Zarzaitine origin and simultaneously compared with the conventional method using a contact tower above the chiller. In the process according to the invention, cold makeup propane was sprayed into the vapor space of the chiller at an initial temperature of 36 F. through several spray nozzles mounted in a suitable header. The conventional contactor using disc and doughnuts as the contacting elements was fed with somewhat warmer propane varying in temperature between about +20 F. and +60 F. As a result the batch chilling time was somewhat longer for the conventional chiller, and the heat transfer duty to warm the makeup propane in the conventional contactor was less severe than in the test chiller using the spray header.

The filter rates of each of the batches was determined by the time needed to draw down the level in the filter feed drum as determined by an automatic recorder. The results show that there was no difference in the slope of the recorder curves showing drop in level versus time in the two cases. This test is 1 considered to be reasonably severe, since the longer chilling time and warmer makeup temperature for the conventional chiller would be expected to produce better crystals and hence better filter rates.

During the test the feed rate was 40 M /hr. for most of the period. The filter area available was 1,500 square feet. No wash solvent was used-only conditioning sprays on the cake ahead of blowback. No filter was hot washed with kerosene. The initial dilution ratio was 1.5. A dewaxing aid was present in the oil-solvent-wax slurry in the chiller consisting of Paraflow I49 and 1S Plexal, fed at a rate equivalent to DJ percent on an active ingredient basis. Final chilling temperature was 30 to 32" C. and dewaxed pour points of 9 to l 2 C. were obtained.

The above data show that important savings can be made by spraying makeup propane or other autorefrigerative solvent into the vapor space of the chiller. The technique of this invention eliminates the deadweight and useless volume of the conventional contactor vessels, each of which weighs about 42,000 lbs. Assuming half this weight of metal is cooled and reheated through a range of F., each batch gives a wasted refrigeration load of 300,000 B.t.u./batch. This is equal to vaporizing about 1,700 lbs. of propane to cool the metal. In addition the difference between the weight of propane vapor in the contactor at the beginning and end of the cycle represents another 2,300 lbs. Thus the use of the conventional contactors add a total of 4,000 lbs. going to the compressor each 30 minute chilling cycle, compared to an average total quantity per batch of 45,000 lbs. Thus the elimination of the conventional contactors results in a 9 percent saving in chilling load.

The nature and advantages of the present invention having thus been fully set forth and illustrated and specific examples of the same given, what is claimed as new, useful and unobvious and desired to be secured by Letters Patent is:

1. The method of chilling a solution of waxy oil in a liquid normally gaseous dewaxing solvent for crystallizing wax in filterable form which comprises introducing said solution into a closed chilling zone under pressure, spraying cold solvent into the vapor space of said chilling zone so as to emerge therein as particles or droplets of a size of about LOGO-6,000 microns, directly contacting said solvent droplets with solvent vapors from said solution whereby the sensible and latent heat of vaporization of the solvent vapors from the said solution preheats the incoming solvent vapor before it is admixed with the solvent solution in the chilling zone and admixing the warmedup solvent with the solution in the chilling zone.

2. The process of claim 1 which includes the step of removing solvent vapors from the chilling zone.

3. The method of chilling a liquid normally gaseous dewaxing solvent solution of waxy oil for the production of filterable wax crystals which comprises introducing the solution into a closed chilling zone, withdrawing solvent vapors from said chilling zone, spraying cold liquid solvent into the vapor space of said chilling zone so as to emerge therefrom as particles or droplets of a size of about 1,000 to 6,000 microns, directly contacting said cold solvent with solvent vaporized from said solution whereby the introduced solvent droplets are brought into temperature equilibrium with the solution before it is admixed therewith, admixing the introduced solvent with the solution in the chilling zone and regulating the amount of solvent introduced and the amount of solvent withdrawn to maintain the optimum diluent ratio.

4. The process of claim 3 in which the solvent is propane.

5. The process of claim 4 in which the propane is sprayed into the vapor space of the chilling zone at substantially the same rate that the propane is removed therefrom so that the liquid level in the said chilling zone remains substantially constant.

'0' l l t I 

2. The process of claim 1 which includes the step of removing solvent vapors from the chilling zone.
 3. The method of chilling a liquid normally gaseous dewaxing solvent solution of waxy oil for the production of filterable wax crystals which comprises introducing the solution into a closed chilling zone, withdrawing solvent vapors from said chilling zone, spraying cold liquid solvent into the vapor space of said chilling zone so as to emerge therefrom as particles or droplets of a size of about 1,000 to 6,000 microns, directly contacting said cold solvent with solvent vaporized from said solution whereby the introduced solvent droplets are brought into temperature equilibrium with the solution before it is admixed therewith, admixing the introduced solvent with the solution in the chilling zone and regulating the amount of solvent introduced and the amount of solvent withdrawn to maintain the optimum diluent ratio.
 4. The process of claim 3 in which the solvent is propane.
 5. The process of claim 4 in which the propane is sprayed into the vapor space of the chilling zone at substantially the same rate that the propane is removed therefrom so that the liquid level in the said chilling zone remains substantially constant. 